Control system for positioning of marine seismic streamers

ABSTRACT

A method of controlling a streamer positioning device ( 18 ) configured to be attached to a marine seismic streamer ( 12 ) and towed by seismic survey vessel ( 10 ) and having a wing and a wing motor for changing the orientation of the wing. The method includes the steps of: obtaining an estimated velocity of the streamer positioning device, calculating a desired change in the orientation of the wing using the estimated velocity of the streamer positioning device, and actuating the wing motor to produce the desired change in the orientation of the wing. The invention also involves an apparatus for controlling a streamer positioning device including means for obtaining an estimated velocity of the streamer positioning device, means for calculating a desired change in the orientation of the wing using the estimated velocity of the streamer positioning device, and means for actuating the wing motor to produce the desired change in the orientation of the wing.

CROSS-REFERENCE TO RELATED APPLICATIONS

Applicant claims priority under 35 U.S.C. § 120 from co-pending Ser. No.11/454,352, filed Jun. 16, 2006, which was a continuation of Ser. No.11/070,614, filed Mar. 2, 2005, now U.S. Pat. No. 7,080,607, which was acontinuation of parent application Ser. No. 09/787,723, filed Jul. 2,2001, now U.S. Pat. No. 6,932,017, which was a 35 U.S.C. § 371 nationalstage filing from Patent Cooperation Treaty application numberPTC/IB99/01590, filed Sep. 28, 1999, which in turn claimed priority fromGreat Britain patent application number 9821277.3, filed Oct. 1, 1998,from which Applicant claims foreign priority under 35 U.S.C. § 119, allof which are incorporated herein by reference. This application is alsorelated to co-pending application Ser. Nos. 11/454,352; 11/454,349 and11/455,042, all filed Jun. 16, 2006, all three of which are alsoincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention related generally to systems for controlling seismic dataacquisition equipment and particularly to a system for controlling amarine seismic streamer positioning device.

A marine seismic streamer is an elongate cable-like structure, typicallyup to several thousand meters long, which contains arrays of seismicsensors, known as hydrophones, and associated electronic equipment alongits length, and which is used in marine seismic surveying. In order toperform a 3D marine seismic survey, a plurality of such streamers aretowed at about 5 knots behind a seismic survey vessel, which also towsone or more seismic sources, typically air guns. Acoustic signalsproduced by the seismic sources are directed down through the water intothe earth beneath, where they are reflected from the various strata. Thereflected signals are received by the hydrophones, and then digitizedand processed to build up a representation of the subsurface geology.

The horizontal positions of the streamers are typically controlled by adeflector, located at the front end or “head” of the streamer, and atail buoy, located at the back end or “tail” of the streamer. Thesedevices create tension forces on the streamer which constrain themovement of the streamer and cause it to assume a roughly linear shape.Cross currents and transient forces cause the streamer to bow andundulate, thereby introducing deviations into this desired linear shape.

The streams are typically towed at a constant depth of approximately tenmeters, in order to facilitate the removal of undesired “ghost”reflections from the surface of the water. To keep the streamers at thisconstant depth, control devices known as “birds”, are typically attachedat various points along each streamer between the deflector and the tailbuoy, with the spacing between the birds generally varying between 200and 400 meters. The birds have hydrodynamic deflecting surfaces,referred to as wings, that allow the position of the streamer to becontrolled as it is towed through the water. When a bird is used fordepth control purposes only, it is possible for the bird to regularlysense its depth using an integrated pressure sensor and for a localcontroller within the bird to adjust the wing angles to maintain thestreamer near the desired depth using only a desired depth valuereceived from a central control system.

While the majority of birds used thus far have only controlled the depthof the streamers, additional benefits can be obtained by using properlycontrolled horizontally steerable birds, particularly by using the typesor horizontally and vertically steerable birds disclosed in ourpublished PCT International Application No. WO 98/28636. The benefitsthat can be obtained by using properly controlled horizontally steerablebirds can include reducing horizontal out-of-position conditions thatnecessitate reacquiring seismic data in a particular area (i.e. in-fillshooting), reducing the chance of tangling adjacent streamers, andreducing the time required to turn the seismic acquisition vessel whenending one pass and beginning another pass during a 3D seismic survey.

It is estimated that horizontal out-of-position conditions reduce theefficiency of current 3D seismic survey operations by 5 and 10%,depending on weather and current conditions. While incidents of tanglingadjacent streamers are relatively rare, when they do occur theyinvariably result in prolonged vessel downtime. The loss of efficiencyassociated with turning the seismic survey vessel will depend in largepart on the seismic survey layout, buy typical estimates range from 5 to10%. Simulations have concluded that properly controlled horizontallysteerable birds can be expected to reduce these types of costs byapproximately 30%.

One system for controlling a horizontally steerable bird, as disclosedin UK Patent GB 2093610 B, is to utilize a manually-operated centralcontrol system to transmit the magnitudes and directions of any requiredwing angle changes to the birds. While this method greatly simplifiesthe circuitry needed within the bird itself, it is virtually impossiblefor this type of system to closely regulate the horizontal positions ofthe birds because it requires manual input and supervision. This becomesa particularly significant issue when a substantial number of streamersare deployed simultaneously and the number of birds that must becontrolled goes up accordingly.

Another system for controlling a horizontally steerable bird isdisclosed in our published PCT International Application No. WO98/28636. Using this type of control system, the desired horizontalpositions and the actual horizontal positions are received from a remotecontrol system and are then used by a local control system within thebirds to adjust the wing angles. The actual horizontal positions of thebirds may be determined every 5 to 10 seconds and there may be a 5second delay between the taking of measurements and the determination ofactual streamer positions. While this type of system allows for moreautomatic adjustment of the bird wing angles, the delay period and therelatively long cycle time between position measurements prevents thistype of control system from rapidly and efficiently controlling thehorizontal position of the bird. A more deterministic system forcontrolling this type of streamer positioning device is thereforedesired.

It is therefore an object of the present invention to provide for animproved method and apparatus for controlling a streamer positioningdevice.

An advantage of the present invention is that the position of thestreamer may be better controlled, thereby reducing the need for in-fillshooting, reducing the chance of streamer tangling, and reducing thetime needed to turn the seismic survey vessel.

Another advantage of the present invention is that noise in marineseismic data associated with streamer position over-correction andstreamer positioning errors can be significantly reduced.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for controlling thepositions of marine seismic streamers in an array of such streamersbeing towed by a seismic survey vessel, the streamers having respectivestreamer positioning devices disposed therealong and each streamerpositioning device having a wing and a wing motor for changing theorientation of the wing so as to steer the streamer positioning devicelaterally, said methods and apparatus involving (a) obtaining anestimated velocity of the streamer positioning devices, (b) for at leastsome of the streamer positioning devices, calculating desired changes inthe orientation of their wings using said estimated velocity, and (c)actuating the wing motors to produce said desired changes in wingorientation.

The invention and its benefits will be better understood with referenceto the detailed description below and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a seismic survey vessel and associatedseismic data acquisition equipment;

FIG. 2 is a schematic horizontal cross-sectional view through a marineseismic streamer and an attached streamer positioning device;

FIG. 3 is a schematic vertical cross-sectional view through the streamerpositioning device from FIG. 2; and

FIG. 4 is a schematic diagram of the local control system architectureof the streamer positioning device from FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a seismic survey vessel 10 is shown towing eight marineseismic streamers 12 that may, for instance, each be 3000 meters inlength. The outermost streamers 12 in the array could be 700 metersapart, resulting in a horizontal separation between the streamers of 100meters in the regular horizontal spacing configuration shown. A seismicsource 14, typically an airgun or an array of airguns, is also shownbeing towed by the seismic survey vessel 10. At the front of eachstreamer 12 is shown a deflector 16 and at the rear of every streamer isshown a tail buoy 20. The deflector 16 is used to horizontally positionthe end of the streamer nearest the seismic survey vessel 10 and thetail buoy 20 creates drag at the end of the streamer farthest from theseismic survey vessel 10. The tension created on the seismic streamer bythe deflector 16 and the tail buoy 20 results in the roughly linearshape of the seismic streamer 12 shown in FIG. 1.

Located between the deflector 16 and the tail buoy 20 are a plurality ofstreamer positioning devices known as birds 18. Preferably the birds 18are both vertically and horizontally steerable. These birds 18 may, forinstance, be located at regular intervals along the streamer, such asevery 200 to 400 meters. The vertically and horizontally steerable birds18 can be used to constrain the shape of the seismic streamer 12 betweenthe deflector 16 and the tail buoy 20 in both the vertical (depth) andhorizontal directions.

In the preferred embodiment of the present invention, the control systemfor the birds 18 is distributed between a global control system 22located on or near the seismic survey vessel 10 and a local controlsystem located within or near the birds 18. The global control system 22is typically connected to the seismic survey vessel's navigation systemand obtains estimates of system wide parameters, such as the vessel'stowing direction and velocity and current direction and velocity, fromthe vessel's navigation system.

The most important requirement for the control system is to prevent thestreamers 12 from tangling. This requirement becomes more and moreimportant as the complexity and the total value of the towed equipmentincreases. The trend in the industry is to put more streamers 12 on eachseismic survey vessel 10 and to decrease the horizontal separationbetween them. To get better control of the streamers 12, horizontalsteering becomes necessary. If the birds 18 are not properly controlled,horizontal steering can increase, rather than decrease, the likelihoodof tangling adjacent streamers. Localized current fluctuations candramatically influence the magnitude of the side control required toproperly position the streamers. To compensate for these localizedcurrent fluctuations, the inventive control system utilizes adistributed processing control architecture and behavior-predictivemodel-based control logic to properly control the streamer positioningdevices.

In the preferred embodiment of the present invention, the global controlsystem 22 monitors the actual positions of each of the birds 18 and isprogrammed with the desired positions of or the desired minimumseparations between the seismic streamers 12. The horizontal positionsof the birds 18 can be derived, for instance, using the types ofacoustic positioning systems described in our U.S. Pat. No. 4,992,990 orin our PCT International Patent Application No. WO 98/21163.Alternatively, or additionally, satelllite-based global positioningsystem equipment can be used to determine the positions of theequipment. The vertical positions of the birds 18 are typicallymonitored using pressure sensors attached to the birds, as discussedbelow.

The global control system 22 preferably maintains a dynamic model ofeach of the seismic streamers 12 and utilizes the desired and actualpositions of the birds 18 to regularly calculate updated desiredvertical and horizontal forces the birds should impart on the seismicstreamers 12 to move them from their actual positions to their desiredpositions. Because the movement of the seismic streamer 12 causesacoustic noise (both from seawater flow past the bird wing structures aswell as cross current flow across the streamer skin itself), it isimportant that the streamer movements be restrained and kept to theminimum correction required to properly position the streamers. Anystreamer positioning device control system that consistentlyoverestimates the type of correction required and causes the bird toovershoot its intended position introduces undesirable noise into theseismic data being acquired by the streamer. In current systems, thistype of over-correction noise is often balanced against the “noise” or“smearing” caused when the seismic sensors in the streamers 12 aredisplaced from their desired positions.

The global control system 22 preferably calculates the desired verticaland horizontal forces based on the behavior of each streamer and alsotakes into account the behavior of the complete streamer array. Due tothe relatively low sample rate and time delay associated with thehorizontal position determination system, the global control system 22runs position predictor software to estimate the actual locations ofeach of the birds 18. The global control system 22 also checks the datareceived from the vessel's navigation system and the data will be filledin if it is missing. The interface between the global control system 22and the local control system will typically operate with a samplingfrequency of at least 0.1 Hz. The global control system 22 willtypically acquire the following parameters from the vessel's navigationsystem: vessel speed (m/s), vessel heading (degrees), current speed(m/s), current heading (degrees), and the location of each of the birdsin the horizontal plane in a vessel fixed coordinate system. Currentspeed and heading can also be estimated based on the average forcesacting on the streamers 12 by the birds 18. The global control system 22will preferably send the following values to the local bird controller:demanded vertical force, demanded horizontal force, towing velocity, andcrosscurrent velocity.

The towing velocity and crosscurrent velocity are preferably“water-referenced” values that are calculated from the vessel speed andheading values and the current speed and heading values, as well as anyrelative movement between the seismic survey vessel 10 and the bird 18(such as while the vessel is turning), to produce relative velocities ofthe bird 18 with respect to the water in both the “in-line” and the“cross-line” directions. Alternatively, the global control system 22could provide the local control system with the horizontal velocity andwater in-flow angle. The force and velocity values are delivered by theglobal control system 22 as separate values for each bird 18 on eachstreamer 12 continuously during operation of the control system.

The “water-referenced” towing velocity and crosscurrent velocity couldalternatively be determined using flowmeters or other types of watervelocity sensors attached directly to the birds 18. Although these typesof sensors are typically quite expensive, one advantage of this type ofvelocity determination system is that the sensed in-line and cross-linevelocities will be inherently compensated for the speed and heading ofmarine currents acting on said streamer positioning device and forrelative movements between the vessel 10 and the bird 18.

FIG. 2 shows a type of bird 18 that is capable of controlling theposition of seismic streamers 12 in both the vertical and horizontaldirections. A bird 18 of this type is also disclosed in out PCTInternational Application No. WO 98/28636. While a number of alternativedesigns for the vertically and horizontally steerable birds 18 arepossible, including those utilizing one full-moving wing with ailerons,three full-moving wings, and four full-moving wings, the independenttwo-wing principal is, conceptually, the simplest and most robustdesign.

In FIG. 2, a portion of the seismic streamer 12 is shown with anattached bird 18. A communication line 24, which may consist of a bundleof fiber optic data transmission cables and power transmission wires,passes along the length of the seismic streamer 12 and is connected tothe seismic sensors, hydrophones 26, that are distributed along thelength of the streamer, and to the bird 18. The bird 18 preferably has apair of independently moveable wings 28 that are connected to rotableshafts 32 that are rotated by wing motors 34 and that allow theorientation of the wings 28 with respect to the bird body 30 to bechanged. When the shafts 32 of the bird 18 are not horizontal, thisrotation causes the horizontal orientation of the wings 28 to change andthereby changes the horizontal forces that are applied to the streamer12 by the bird.

The motors 34 can consist of any type of device that is capable ofchanging the orientation of the wings 28, and they are preferably eitherelectric motors or hydraulic actuators. The local control system 36controls the movement of the wings 28 by calculating a desired change inthe angle of the wings and then selectively driving the motors 34 toeffectuate this change. While the preferred embodiment depicted utilizesa separate motor 34 for each wing 28, it would be also be possible toindependently move the wings 28 using a single motor 34 and aselectively actuatable transmission mechanism.

When the bird 18 uses two wings 28 to produce the horizontal andvertical forces on the streamer 12, the required outputs of the localcontrol system 36 are relatively simple, the directions and magnitudesof the wing movements required for each of the wings 28, or equivalentlythe magnitude and direction the motors 34 need to be driven to producethis wing movement. While the required outputs of the local controlsystem 36 for such a two full moving wing design is quite simple, thestructure and operation of the overall system required to coordinatecontrol of the device is relatively complicated.

FIG. 3 shows a schematic vertical cross-sectional view through thestreamer positioning device shown in FIG. 2 that will allow theoperation of the inventive control system to be described in moredetail. The components of the bird 18 shown in FIG. 3 include the wings28 and the body 30. Also shown in FIG. 3 are horizontal coordinate axis38 and a vertical coordinate axis 40. During operation of the streamerpositioning control system, the global control system 22 preferablytransmits, at regular intervals (such as every five seconds) a desiredhorizontal force 42 and a desired vertical force 44 to the local controlsystem 36.

The desired horizontal force 42 and the desired vertical force 44 arecombined with the local control system 36 to calculate the magnitude anddirection of the desired total force 46 that the global control system22 has instructed the local control system to apply to the streamer 12.The global control system 22 could alternatively provide the magnitudeand direction of the desired total force 46 to the local control system36 instead of the desired horizontal force 42 and the desired verticalforce 44.

While the desired horizontal force 42 and the desired vertical force 44are preferably calculated by the global control system 22, it is alsopossible for the local control system 36 in the inventive control systemto calculate one or both of these forces using a localizeddisplacement/force conversion program. This type of localized conversionprogram may, for instance, use a look-up table or conversion routinethat associates certain magnitudes and directions of vertical orhorizontal displacements with certain magnitudes and directions ofchanges in the vertical or horizontal forces required. Using this typeof embodiment, the global control system 22 can transmit locationinformation to the local control system 36 instead of force information.Instead of the desired vertical force 44, the global control system 22can transmit a desired vertical depth and the local control system 36can calculate the magnitude and direction of the deviation between thedesired depth and the actual depth. Similarly, instead of transmitting adesired horizontal force 42, the global control system 22 can transmitthe magnitude and direction of the displacement between the actualhorizontal position and the desired horizontal position of the bird 18.One advantage to this alternative type of system is that the requiredvertical force can be rapidly updated as the local control systemreceives updated depth information from the integrated pressure sensor.Other advantages of this type of alternative system include reducingcommunication traffic on the communication line 24 and simplifying theprogramming needed to convert the measured vertical and/or horizontaldisplacements into corresponding forces to be applied by the birds 18.

When the local control system 36 has a new desired horizontal force 42and desired vertical force 44 to be applied, the wings 28 will typicallynot be in the proper orientation to provide the direction of the desiredtotal force 46 required. As can be seen in FIG. 3, the wings 28introduce a force into the streamer 12 along an axis perpendicular tothe rotational axis of the wings 28 and perpendicular to the streamer.This force axis 48 is typically not properly aligned with the desiredtotal force 46 when new desired horizontal and vertical force values arereceived from the global control system 22 or determined by the localcontrol system 36 and some rotation of the bird 18 is required beforethe bird can produce this desired total force 46. As can be seen, theforce axis 48 is directly related to the bird roll angle, designated inFIG. 3 as φ.

The local control system 36 optimizes the control process by projectingthe desired total force 46 onto the force axis 48 (i.e. multiplying themagnitude of the desired total force by the cosine of the deviationangle 50) to produce an intermediate desired force 52 and then adjustingthe wing common angle α (the angle of the wings with respect to the birdbody 30, or the average angle if there is a non-zero splay angle) toproduce this magnitude of force along the force axis. The calculateddesired common wing angle is compared to the current common wing angleto calculate a desired change in the common wing angle and the wingmotors 34 are actuated to produce this desired change in the orientationof the wings.

A splay angle is then introduced into the wings 28 to produce arotational movement in the bird body 30 (i.e. to rotate the force axis48 to be aligned with the desired total force 46). The splay angle isthe difference between the angles of the wings 28 with respect to thebird body 30. As the bird body 30 rotates and the force axis 48 becomesmore closely aligned with the desired total force 46, the bird rollangle and the bird roll angular velocity are monitored, the splay angleis incrementally reduced, and the common angle is incrementallyincreased until the intermediate desired force 52 is in the samedirection and of the same magnitude as the desired total force. Thelocal control system 36 carefully regulates the splay angle to ensurethat the streamer is stable in roll degree of freedom. The calculatedcommon wing angle and the splay angle are also regulated by the localcontrol system 36 to prevent the wings 28 from stalling and to ensurethat the splay angle is prioritized.

When using the type of birds described in our published PCTInternational Application No. WO 98/28636, where the bird 18 is rigidlyattached, and cannot rotate with respect, to the streamer 12, it isimportant for the control system to take the streamer twist intoaccount. If this is not taken into account, the bird 18 can use all ofits available splay angle to counter the twist in the streamer 12. Thebird 18 will then be unable to reach the demanded roll angle and thegenerated force will decrease. The inventive control system incorporatestwo functions for addressing this situation; the anti-twist function andthe untwist function.

In the anti-twist function, the streamer twist is estimated byweightfunction filtering the splay angle measurements instead of simplyaveraging the splay angle measurements to improve the bandwidth of theestimation. The anti-twist function engages when the estimated twist hasreached a critical value and it then overrides the normal shortest pathcontrol of the calculated roll angle. The anti-twist function forces thebird 18 to rotate in the opposite direction of the twist by adding+/−180 degrees to the demanded roll angle. Once the twist has beenreduced to an acceptable value, the anti-twist function disengages andthe normal shortest path calculation is continued.

The untwist function is implemented by the global control system 22which monitors the splay angle for all of the birds 18 in each streamer12. At regular intervals or when the splay angle has reached a criticalvalue, the global control system 22 instructs each local control system36 to rotate each bird 18 in the opposite direction of the twist. Thenumber of revolutions done by each bird 18 is monitored and the untwistfunction is disengaged once the twist has reached an acceptable level.FIG. 4 is a schematic diagram of the architecture of the local controlsystem 36 for the bird 18. The local control system 36 consists of acentral processor unit 54, having EEPROM 56 and RAM 58 memory, aninput/output subsystem 60 that is connected to a pair of motor drivers62, and an analog to digital conversion unit 66. The motor drivers 62are connected to and actuate the wing motors 34 to produce the desiredchange the orientation of the wings 28 with respect to the bird body 30.

The wing motor 34/wing 28 units are also connected to wing positionindicators 64 that sense the relative positions of the wings and providemeasurements to the analog to digital conversion unit 66 which convertsthe analog wing position indicator 64 measurements into digital formatand conveys these digital values to the central processor unit 54.Various types of wing position indicators 64 can be used, includingresistive angle or displacement sensors inductive sensors, capacitivesensors, hall sensors, or magneto-restrictive sensors.

A horizontal accelerometer 68 and a vertical accelerometer 70, placed atright angles with respect to one another, are also connected to theanalog to digital conversion unit 66 and these accelerometers conveymeasurements that allow the central processor unit 54 to determine theroll angle and roll rate of the bird 18. An angular velocity vibratingrate gyro (rategyro) can also be used to measure the roll rate of thebird 18. A temperature sensor 72 is connected to the analog to digitalconversion unit 66 to provide temperature measurements that allow thehorizontal accelerometer 68 and the vertical accelerometer 70 to becalibrated.

A pressure sensor 74 is also connected to the analog to digitalconversion unit 66 to provide the central processor unit 54 withmeasurements of the water pressure at the bird 18. To calculate anappropriate depth value, the measured pressure values must be filteredto limit the disturbance from waves. This is done in the inventivecontrol system with a weightfunction filter that avoids the large phasedisplacements caused by mean value filters. Instead of using aninstantaneous depth value or simply calculating an average depth valueover a given period of time (and thereby incorporating a large phasedisplacement into the depth value), the inventive control system uses adifferently weighted pressure filtering scheme. First the pressurevalues are transformed into depth values by dividing the pressure sensorreading by the seawater density and gravitational acceleration. Thesedepth values are then filtered using a weight function filter. Typicalincremental weighting functions values range from 0.96 to 0.90 (sampleweights of 1.0, 0.9, 0.81, 0.729, etc.) and the filter will typicallyprocess depth values received over a period of at least 100 seconds.

The central processor unit 54 is also connected to a RS485communications unit 76 that allows information to be exchanged betweenthe local control system 36 and the global control system 22 over thecommunication line 24 that passes through the streamer 12. The RS485 busmay, for instance, utilize Neuron chips that communicate using a LocalOperating Network protocal to control the data transfer.

Preferably, the central processor unit 54 and associated componentscomprise a MicroChip 17C756 processor. This type of microprosessor hasvery low power requirements, a dual UART on-chip, 12-channel, 10 bit ADCon-chip, 908×8 RAM, 16 k×16 ROM, and 50 digital I/O channels. Thesoftware running on the central processor unit 54 will typically consistof two units, the local control unit and the hardware control unit. Itis typically not possible to pre-load both of these program units intothe EEPROM 56 and it is possible to update these program units withouthaving to open the bird 18. The on-chip memory may thus only initiallycontain a boot-routine that enables the loading of software units intothe external memory via the RS485 communication unit 76. The externalprogram memory (EEPROM 56) will typically be a non-volatile memory sothat these program units do not have to be re-loaded after every powerdown.

The central processor unit 54 must be able to run the local controlsystem software fast enough to secure the sampling frequency needed foreffective local bird control. This may mean, for instance, a sample rateof 10 Hz, which may be 10 to 100 times faster than the sample rate ofthe communications between the global control system 22 and the localcontrol system 36. As discussed above, the central processor unit 54will also receive data from sensors attached to the bird 18. The sensedvalues include bird roll angle, bird roll angular velocity (roll rate),the wing angles, and the static pressure of the water. These values aretypically delivered to the central processor unit 54 at a sample rate ofat least 10 Hz. The following values may be transmitted from the localcontrol system 36 to the global control system 22 using the RS485communication unit 76: the measured roll angle, the measured roll rate,the measured wing angles, the measured water pressure, the calculateddepth, and the calculated wing forces.

The system has been designed with a redundant communication system toincrease its overall reliability. The bird 18 will typically have abackup communications channel, such as by overlaying a backup controlsignal on top of the power line current. This backup communicationschannel is particularly important because in the event of loss ofcommunications to the bird 18 there would otherwise be no method forinstructing the bird 18 to bring the streamer 12 to surface so thedefective communications equipment can be repaired or replaced.

In contrast to previous streamer position device control systems, thepresent control system converts the desired horizontal force 42 and thedesired vertical force 44 into a desired roll angle φ and a desiredcommon wing angle α by deterministic calculations, rather than using an“incremental change/measured response/further incremental change basedon measured response” type of feedback control circuit. The desired rollangle φ can be calculated in the manner discussed in the text describingFIG. 3 above. The magnitude of the force F imparted by the wings 28along the force axis 48 can, for instance, be deterministicallycalculated using the following formula:F=0.5 ρAC _(L)(V _(tow)cos(α) . . . V _(current)sin(α))²

where: ρ=water density;

A=wing area;

C_(L)=wing lift coefficient;

α=common wing angle;

V_(tow)=towing velocity; and

V_(current)=crosscurrent volocity.

A similar deterministic calculation could be made using a calculatedcoefficient that incorporates the towing velocity of the bird 18. A gainfactor GF, for instance, could be calculated as follows:GF=0.5 ρAC _(L)(V _(tow))²

which could be simply multiplied by cos(α)² to estimate the force thatwould be applied for a given common angle.

One of the beneficial elements of the inventive control system is thatthe desired change in the orientation of the wing 28 is calculated usingan estimate of the velocity of the bird 18 rather than simply relying ona feedback-loop type of control system that operates in the same mannerregardless of the vessel speed. Because the force produced by wing 28 isproportional to the velocity of the device squared, a much more precisecalculation of the desired change in the wing orientation can be made byusing an estimate of the device velocity.

The inventive control system is based on shared responsibilities betweenthe global control system 22 located on the seismic survey vessel 10 andthe local control system 36 located on the bird 18. The global controlsystem 22 is tasked with monitoring the positions of the streamers 12and providing desired forces or desired position information to thelocal control system 36. The local control system 36 within each bird 18is responsible for adjusting the wing splay angle to rotate the bird tothe proper position and for adjusting the wing common angle to producethe magnitude of total desired force required.

The inventive control system will primarily operate in two differentcontrol modes: a feather angle control mode and a turn control mode. Inthe feather angle control mode, the global control system 22 attempts tokeep each streamer in a straight line offset from the towing directionby a certain feather angle. The feather angle could be input eithermanually, through use of a current meter, or though use of an estimatedvalue based on the average horizontal bird forces. Only when thecrosscurrent velocity is very small will the feather angle be set tozero and the desired streamer positions be in precise alignment with thetowing direction.

The turn control mode is used when ending one pass and beginning anotherpass during a 3D seismic survey, sometimes referred to as a “linechange”. The turn control mode consists of two phases. In the first partof the turn, every bird 18 tries to “throw out” the streamer 12 bygenerating a force in the opposite direction of the turn. In the lastpart of the turn, the birds 18 are directed to go to the positiondefined by the feather angle control mode. By doing this, a tighter turncan be achieved and the turn time of the vessel and equipment can besubstantially reduced. Typically during the turn mode adjacent streamerswill be depth separated to avoid possible entaglement during the turnand will be returned to a common depth as soon as possible after thecompletion of the turn. The vessel navigation system will typicallynotify the global control system 22 when to start throwing the streamers12 out, and when to start straightening the streamers.

In extreme weather conditions, the inventive control system may alsooperate in a streamer separation control mode that attempts to minimizethe risk of entanglement of the streamers. In this control mode, theglobal control system 22 attempts to maximize the distance betweenadjacent streamers. The streamers 12 will typically be separated indepth and the outermost streamers will be positioned as far away fromeach other as possible. The inner streamers will then be regularlyspaced between these outermost streamers, i.e. each bird 18 will receivedesired horizontal forces 42 or desired horizontal position informationthat will direct the bird 18 to the midpoint position between itsadjacent streamers.

While the embodiment of the inventive control system described above isshown in connection with a “bird” type of streamer positioning device,it will be readily understood that the control system method andapparatus may also be used in connection with streamer positioningdevices that are characterized as “deflectors” or steerable “tail buoys”because they are attached to either the front end or the back end of thestreamer 12.

The present invention includes any novel feature or novel combination offeatures disclosed herein, either explicitly or implicitly.

1. A method comprising: (a) towing an array of streamers each having aplurality of streamer positioning devices there along, at least one ofthe streamer positioning devices comprising wings; and (b) positioningthe at least one streamer positioning device by converting a desiredforce on the streamer positioning device into a desired roll angle and adesired common wing angle by one or more deterministic calculations. 2.The method of claim 1 wherein a force F imparted by the wings of thestreamer positioning device along a force axis is deterministicallycalculated using the formula:F=0.5ρAC _(L)(V _(tow)cos(α) . . . V _(current)sin(α))² where: ρ=waterdensity; A=wing area; C_(L)=wing lift coefficient; α=common wing angle;V_(tow)=towing velocity; and V_(current)=crosscurrent velocity.
 3. Themethod of claim 1 wherein a gain factor GF is deterministicallycalculated that incorporates towing velocity of the streamer positioningdevice, and wherein the gain factor is multiplied by cos(α)² to estimatethe force that would be applied to the streamer positioning device for agiven common angle α.
 4. The method of claim 3 wherein the gain factoris calculated as follows:GF=0.5ρAC _(L)(V _(tow))² where: ρ=water density; A=wing area;C_(L)=wing lift coefficient; α=common wing angle; V_(tow)=towingvelocity;
 5. The method of claim 1 wherein the towing an array ofstreamers comprises towing the streamers using a seismic survey vessel.6. The method of claim 5 comprising towing one or more seismic sourcesusing the seismic survey vessel.
 7. The method of claim 6 comprisingdirecting acoustic signals produced by the seismic sources through waterinto the earth beneath the water, producing reflected signals fromvarious strata in the earth.
 8. The method of claim 7 comprisingreceiving the reflected signals by sensors in the streamers.
 9. Themethod of claim 8 comprising digitizing the received reflected signalsand processing the digitized signals to build up a representation of thevarious strata in the earth.
 10. The method of claim 1 wherein the oneor more deterministic calculations are performed by a controllerselected from a global controller, a plurality of local controllersassociated with the streamer positioning devices, and combinationsthereof.
 11. An apparatus comprising: (a) an array of streamers eachhaving a plurality of streamer positioning devices there along, at leastone of the streamer positioning devices comprising wings having a commonwing angle; and (b) a calculation unit calculating a desired roll angleand a desired common wing angle of the at least one streamer positioningdevice from a desired force on the streamer positioning device by adeterministic calculation.
 12. The apparatus of claim 11 wherein thecalculation unit calculates deterministically a force F imparted by thewings of the streamer positioning device along a force axis using theformula:F=0.5 ρAC _(L)(V _(tow)cos(α) . . . V _(current)sin(α))² where: ρ=waterdensity; A=wing area; C_(L)=wing lift coefficient; α=common wing angle;V_(tow)=towing velocity; and V_(current)=crosscurrent velocity.
 13. Theapparatus of claim 11 wherein the calculation unit calculates a gainfactor GF deterministically that incorporates towing velocity of thestreamer positioning device, and multiplies the gain factor by cos(α)²to estimate the force that would be applied to the streamer positioningdevice for a given common angle α.
 14. The apparatus of claim 13 whereinthe gain factor is calculated as follows:GF=0.5 ρAC _(L)(V _(tow))² where: ρ=water density; A=wing area;C_(L)=wing lift coefficient; α=common wing angle; V_(tow)=towingvelocity.
 15. The apparatus of claim 11 comprises a siesmic surveyvessel towing the array of streamers.
 16. The apparatus of claim 15comprising one or more seismic sources towed by the seismic surveyvessel.
 17. The apparatus of claim 16 where in the one or more seismicsources comprise one or more air guns.
 18. The apparatus of claim 17wherein the streamers comprise sensors for receiving reflected signals.19. The apparatus of claim 18 comprising a reflected signal digitizingand processing unit for digitizing the received reflected signals andprocessing the digitized signals to build up a representation of thevarious strata in the earth.
 20. An apparatus comprising: (a) an arrayof streamers each having a plurality of streamer positioning devicesthere along, at least one of the streamer positioning devices comprisingwings having a common wing angle; and (b) a calculation unit calculatinga desired roll angle and a desired common wing angle of the at least onestreamer positioning device from a desired force on the streamerpositioning device by a deterministic calculation, wherein thecalculation unit calculated deterministically a force F imparted by thewings of the streamer positioning device along a force axis using theformula:F=0.5ρAC _(L)(V _(tow)cos(α) . . . V _(current)sin(α))² where: ρ=waterdensity; A=wing area; C_(L)=wing lift coefficient; α=common wing angle;V_(tow)=towing velocity; and V_(current)=crosscurrent velocity.